|Matter | Pages 62-63 ||
Abright visitor blazed through our solar system more than four millennia ago as Egyptian laborers built the great pyramids. The laborers may have gazed at this "hairy star," wondering at its swift pace through the heavens and its shifting shape from night to night. When this same visitor returned in the late twentieth century from a cold hibernation far beyond the orbit of Pluto, we called it Comet Hale-Bopp. The comet graced the sky for months, easily visible without a telescope even from light-polluted cityscapes. In the countryside it was a breathtaking sight, with a gauzy tail stretching millions of miles through interplanetary space.
More people may have seen Comet Hale-Bopp than any other comet in history. Those who followed its journey closely were treated to a vivid illustration of the ever-changing nature of matter in our universe. Since its previous passage near Earth in the twenty-third century B.C., the comet had spent most of its time drifting as a quiet lump on a cigar-shaped orbit 50 billion miles long. Light from the distant Sun was too feeble to heat the comet's skin. Its ices and dust were locked in a frozen embrace, relics of the interstellar material that formed our solar system nearly 5 billion years ago.
As the comet neared the inner planets, the Sun's gravity tugged it inward at a quickening pace. The growing warmth transformed the comet's surface. The outermost layers--consisting mostly of frozen water, carbon dioxide, methane, and ammonia--started to evaporate into space. This made the comet shrink, by the same process that withers ice cubes to sour-tasting nubs when you leave them in your freezer too long. (In this process, called "sublimation," solids transform directly into gas without passing through a liquid state.) Gaseous geysers erupted at weak spots on the surface, expelling dust and spewing vaporized ices into a puffy cloud, or coma, around the comet's nucleus. The dust came from impurities in the comet's ices that were left behind when the ice sublimated. This coma reflected enough sunlight to become visible through telescopes on Earth when the comet was still more than 500 million miles away, beyond the orbit of Jupiter. On the evening of July 22, 1995, the professional astronomer Alan Hale and amateur astronomer Thomas Bopp independently spotted the slowly moving fuzzy blob within minutes of each other. Their discovery gave the comet its catchy hyphenated name.
Comet Hale-Bopp's coma grew larger as the comet fell toward the Sun. Gradually, the combined influence of sunlight and solar wind--a steady blast of charged particles that blow outward through the solar system--pushed the dust and gas into a stream pointing away from the Sun. This tail, the classic signature of a comet, grew as long as 50 million miles when Comet Hale-Bopp ventured closest to the Sun in April 1997. Careful viewers may have noted that the comet displayed two distinct tails. Most obvious was the bright dust tail, illuminated by the Sun's rays as it swept away from the comet's nucleus like a witch's broom. The gentle pressure of sunlight forced these tiny grains outward along curved paths as the comet moved through space. In dark skies a second tail also appeared: an ethereal gas tail, shining with a faint blue light emitted by the gas particles themselves. The electrically charged solar wind propelled this gas on a straight path away from the Sun.
On certain nights you may have seen stars shining through Comet Hale-Bopp's tail as if it wasn't there at all. Indeed, comet tails are among the wispiest collections of matter imaginable. The puffs of gas and dust stretch into a huge volume of space, just as a small cloud in the sky holds only a cupful of water sprayed into tiny droplets of vapor. Reflected sunlight makes both the cloud and the comet tail appear much more substantial than they really are. If we could have compressed Comet Hale-Bopp's tail to the density of the air we breathe, its 50-million-mile length would have fit into a cube no more than 10 miles on a side. The Harvard astronomer Fred Whipple, the first to characterize comets as "dirty snowballs," succinctly described a comet's tail as "the most that has ever been made of the least."